McAdams grew up in Ontario, acting in the Original Kids Theatre Company and in high school plays. Although she was planning to pursue a cultural studies degree, McAdams was convinced to study theatre instead. She entered the drama program at York University, Toronto, earning a B.F.A. (2001) while acting professionally with the Necessary Angel Theatre Company. In 2001 she debuted on-screen in a minor role on the Disney Channel’s television show The Famous Jett Jackson. The following year she made her first feature film, My Name Is Tanino, a comedy about a summer romance.

After moving to Los Angeles, McAdams was quickly cast as a lead in The Hot Chick (2002), playing a shallow high school girl who switches bodies with a male career criminal (played by Rob Schneider). She next made headlines as the malicious popular girl Regina George in Mean Girls (2004), a comedy written and directed by Tina Fey that was a commercial and critical success. Demonstrating her versatility, in 2004 McAdams starred as Allie, a vivacious upper-class woman who must choose between her wealthy fiancé (James Marsden) and her first love (Ryan Gosling), in the adaptation of Nicholas Sparks’s best-selling novel The Notebook. McAdams played another love interest in the comedy Wedding Crashers (2005), but she continued to embrace movies in all genres, starring in the thriller Red Eye (2005) and the crime drama State of Play (2009). In 2009 she appeared opposite Eric Bana in The Time Traveler’s Wife, a love story based on Audrey Niffenegger’s novel of the same name. She also featured in Sherlock Holmes (2009) as Irene Adler, a loosely interpreted version of one of the few love interests to cross Holmes’s path in the detective series by Sir Arthur Conan Doyle, upon which the film was based. In 2010 McAdams portrayed a plucky television producer in the comedy Morning Glory.